The news marks a further deterioration in relations between the Independent Parliamentary Standards Authority and the MPs it is supposed to regulate, after Ipsa recently published part-redacted details of MPs' landlords.

The MPs are challenging a rule that they must repay profits made on their second homes between May 2010 and August 2012, even if they have not sold the property.

Seventy MPs are thought to have claimed nearly £1million in mortgage interest payments over the period.

The change was introduced in the wake of the 2009 MPs expenses scandal to stop MPs profiting from claiming mortgage interest payments on second homes.

Ipsa gave the MPs until November 30 - three months after the change in the rules took place - to repay the sums owed.

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But in a letter sent last month, seen by The Daily Telegraph, the MPs’ lawyer Peter Vaines asked Ipsa to delay the deadline and threatened the regulator with a judicial review in the High Court.

Mr Vaines, a partner of Squires Sanders, said that there is no “statutory or any other authority for Ipsa to require any such payment regarding such notional gain”.

The threat of having to repay the sums was “has created unjustifiable anxiety to members”, Mr Vaines added.

He argued that MPs should be let off because “when the members accepted the mortgage interest subsidy there was no requirement of a payment to Ipsa of a proportion of any notional gain”.

He continued: “In our view it is unfair and unreasonable for Ipsa to introduce a mortgage interest subsidy scheme on certain terms and, when those terms have been accepted and the member has changed his financial circumstances accordingly, for those rules to be altered substantially to his disadvantage.”

Mr Vaines said he was advised “that members who are in that position are entitled to challenge any such claim by Ipsa by way of judicial review in the high court”.

He added: “Some members would need to sell the properties in question if this charge is to be paid.

“If the claims are indeed unauthorised, as we believe it to be, such a sale would have been unnecessary at a great deal of personal disturbance and possibly financial loss would arise in consequence for which a claim for compensation would be competent not only on behalf of a member but also for any other party with an interest in the property.”

He added that there were “numerous other issues here including but not limited to the capital gains tax implications on the ultimate sale of the property, as well as the rights of spouses, children and others who may have an interest in the property”.

Mr Vaines pointed out that Ipsa had now way to force a member to pay up “if a member simply refuses to make payments of this amount on the grounds that he believes it is unlawful or at least beyond the realms of Ipsa”.

Adam Afriye MP, chairman of the Commons members' expenses committee, who has been critical of the Ipsa regime, said that it was only right that Ipsa was “answerable in law for the fairness and legality of its actions and the rules created by the board and executives

“It is no secret that I believe IPSA is wasting taxpayers’ money and creating a two tier parliament which favours those with independent means.

“Something is clearly wrong when Ipsa's schemes are costing about £9m to administrate just £19m worth of expenses and 90% of MPs are subsidising their work from their own pockets.”

Last night Mr Vaines declined to comment. A spokesman for Ipsa said he would not “confirm or deny legal letters”. He added: “The rules on mortgage interest payments were set out clearly in March 2010 and haven’t changed since.

“Any MP who continued to claim mortgage interest during the transition period, did so on the basis that they would repay the taxpayers’ share of any capital gain.”

[blob] MPs are under fresh pressure to let the Commons sleaze watchdog investigate their private lives, it was disclosed.

The Standards and Privileges Committee has renewed its call for rules to cover all behaviour that "significantly damages the reputation of parliament".

MPs blocked a similar move in March, with Tory Charles Walker warning during a debate that it would result in a focus on "the bedroom and the bottle". The then Leader of the House, Sir George Young - now Chief Whip - also suggested it could breach politicians' human rights.

As a result the Code of Conduct makes clear that the standards commissioner may not look into allegations relating to members' "purely private and personal lives". Instead MOPs

However, in a report published on Thursday, the committee set the stage for another showdown by insisting the situation was "unsatisfactory and uncertain".

Letter sent to Ipsa on 21 November 2012 by Peter Vaines, Partner, Squires Sanders UK LLP

We are instructed on behalf of honourable members who are concerned about certain aspects of the mortgage interest subsidy scheme details of which are set out in annex 3 for the third and fourth editions of the MPs expenses scheme.

The particular area of concern is that when the member has received the benefit of a mortgage interest subsidy it is suggested that he was required to have the relevant property valued in August 2012 and to pay to Ipsa part of the notional gain that may have arisen during the transitional period in proportion to the amount of mortgage subsidy. It is said this payment must be made by 30 November 2012.

We do not believe there is any statutory or other authority for Ipsa to require any payment regarding such notional gain and this suggestion has created unjustifiable anxiety to members.

When the members accepted the mortgage interest subsidy there was no requirement for a payment to Ipsa of a proportion of any notional gain. This concept was subsequently introduced in April 2011 version the MPs’ expenses scheme.

In our view it is unfair and unreasonable for Ipsa to introduce a mortgage interest subsidy scheme on certain terms and when those terms have been accepted and the member has changed his financial circumstances accordingly for those rules to be altered substantially to his disadvantage.

We were advised that members who are in that position are entitled to challenge any such claim by Ipsa by way of judicial review in the high court.

Some members would need to sell the properties in question if this charge is to be paid. If the claims are indeed unauthorised, as we believe it to be, such a sale would have been unnecessary at a great deal of personal disturbance and possibly financial loss would arise in consequence for which a claim for compensation would be competent not only on behalf of a member but also for any other party with an interest in the property.

We understand that each of the members concerned will be writing to you separately concerning the operation of this scheme but it seems to us that the most sensible course of action would be for the 30 November deadline to be deferred for a reasonable period so this matter can be resolved.

We should be grateful if you would confirm you will extend the November deadline at your earliest convenience. There are numerous other issues here including but not limited to the capital gains tax implications on the ultimate sale of the property, as well as the rights of spouses, children and others who may have an interest in the property.

There are also aspects of financial services legislation which would seem to apply to these circumstances and give rise to serious concern. It is also relevant that there is no enforcement procedure if a member simply refuseS to make payments of this amount on the grounds that he believes it is unlawful or at least beyond the realms of Ipsa for such a claim to be made.

We appreciate that the mortgage interest subsidy scheme has now come to an end but we will continue to deal with the matter as regards to these members who participated in this scheme during the transitional period and are faced with the unpleasant decisions by the end of this month.

Our instructions are to represent those who have been placed in this position and to protect their position and that of their partners and spouses. We do appreciate the sensitivity of this matter and suggest it may be useful for us to have a without prejudice meeting in the near future to discuss a mutually acceptable solution to this unfortunate issue.